/**
* @file workqueue.h
* @author Nat Goodspeed
* @date 2021-09-30
* @brief Queue used for inter-thread work passing.
*
* $LicenseInfo:firstyear=2021&license=viewerlgpl$
* Copyright (c) 2021, Linden Research, Inc.
* $/LicenseInfo$
*/
#if ! defined(LL_WORKQUEUE_H)
#define LL_WORKQUEUE_H
#include "llinstancetracker.h"
#include "threadsafeschedule.h"
#include <chrono>
#include <functional> // std::function
#include <queue>
#include <string>
#include <utility> // std::pair
#include <vector>
namespace LL
{
/**
* A typical WorkQueue has a string name that can be used to find it.
*/
class WorkQueue: public LLInstanceTracker<WorkQueue, std::string>
{
private:
using super = LLInstanceTracker<WorkQueue, std::string>;
public:
using Work = std::function<void()>;
private:
using Queue = ThreadSafeSchedule<Work>;
// helper for postEvery()
template <typename Rep, typename Period, typename CALLABLE>
class BackJack;
public:
using TimePoint = Queue::TimePoint;
using TimedWork = Queue::TimeTuple;
using Closed = Queue::Closed;
/**
* You may omit the WorkQueue name, in which case a unique name is
* synthesized; for practical purposes that makes it anonymous.
*/
WorkQueue(const std::string& name = std::string());
/**
* Since the point of WorkQueue is to pass work to some other worker
* thread(s) asynchronously, it's important that the WorkQueue continue
* to exist until the worker thread(s) have drained it. To communicate
* that it's time for them to quit, close() the queue.
*/
void close();
/*---------------------- fire and forget API -----------------------*/
/// fire-and-forget, but at a particular (future?) time
template <typename CALLABLE>
void post(const TimePoint& time, CALLABLE&& callable)
{
// Defer reifying an arbitrary CALLABLE until we hit this method.
// All other methods should accept CALLABLEs of arbitrary type to
// avoid multiple levels of std::function indirection.
mQueue.push(TimedWork(time, std::move(callable)));
}
/// fire-and-forget
template <typename CALLABLE>
void post(CALLABLE&& callable)
{
// We use TimePoint::clock::now() instead of TimePoint's
// representation of the epoch because this WorkQueue may contain
// a mix of past-due TimedWork items and TimedWork items scheduled
// for the future. Sift this new item into the correct place.
post(TimePoint::clock::now(), std::move(callable));
}
/**
* Launch a callable returning bool that will trigger repeatedly at
* specified interval, until the callable returns false.
*
* If you need to signal that callable from outside, DO NOT bind a
* reference to a simple bool! That's not thread-safe. Instead, bind
* an LLCond variant, e.g. LLOneShotCond or LLBoolCond.
*/
template <typename Rep, typename Period, typename CALLABLE>
void postEvery(const std::chrono::duration<Rep, Period>& interval,
CALLABLE&& callable);
/*------------------------- handshake API --------------------------*/
/**
* Post work to another WorkQueue to be run at a specified time,
* requesting a specific callback to be run on this WorkQueue on
* completion.
*
* Returns true if able to post, false if the other WorkQueue is
* inaccessible.
*/
// Apparently some Microsoft header file defines a macro CALLBACK? The
// natural template argument name CALLBACK produces very weird Visual
// Studio compile errors that seem utterly unrelated to this source
// code.
template <typename CALLABLE, typename FOLLOWUP>
bool postTo(WorkQueue::weak_t target,
const TimePoint& time, CALLABLE&& callable, FOLLOWUP&& callback)
{
// We're being asked to post to the WorkQueue at target.
// target is a weak_ptr: have to lock it to check it.
auto tptr = target.lock();
if (! tptr)
// can't post() if the target WorkQueue has been destroyed
return false;
// Here we believe target WorkQueue still exists. Post to it a
// lambda that packages our callable, our callback and a weak_ptr
// to this originating WorkQueue.
tptr->post(
time,
[reply = super::getWeak(),
callable = std::move(callable),
callback = std::move(callback)]
()
{
// Call the callable in any case -- but to minimize
// copying the result, immediately bind it into a reply
// lambda. The reply lambda also binds the original
// callback, so that when we, the originating WorkQueue,
// finally receive and process the reply lambda, we'll
// call the bound callback with the bound result -- on the
// same thread that originally called postTo().
auto rlambda =
[result = callable(),
callback = std::move(callback)]
()
{ callback(std::move(result)); };
// Check if this originating WorkQueue still exists.
// Remember, the outer lambda is now running on a thread
// servicing the target WorkQueue, and real time has
// elapsed since postTo()'s tptr->post() call.
// reply is a weak_ptr: have to lock it to check it.
auto rptr = reply.lock();
if (rptr)
{
// Only post reply lambda if the originating WorkQueue
// still exists. If not -- who would we tell? Log it?
try
{
rptr->post(std::move(rlambda));
}
catch (const Closed&)
{
// Originating WorkQueue might still exist, but
// might be Closed. Same thing: just discard the
// callback.
}
}
});
// looks like we were able to post()
return true;
}
/**
* Post work to another WorkQueue, requesting a specific callback to
* be run on this WorkQueue on completion.
*
* Returns true if able to post, false if the other WorkQueue is
* inaccessible.
*/
template <typename CALLABLE, typename FOLLOWUP>
bool postTo(WorkQueue::weak_t target,
CALLABLE&& callable, FOLLOWUP&& callback)
{
return postTo(target, TimePoint::clock::now(), std::move(callable), std::move(callback));
}
/*--------------------------- worker API ---------------------------*/
/**
* runUntilClose() pulls TimedWork items off this WorkQueue until the
* queue is closed, at which point it returns. This would be the
* typical entry point for a simple worker thread.
*/
void runUntilClose();
/**
* runPending() runs all TimedWork items that are ready to run. It
* returns true if the queue remains open, false if the queue has been
* closed. This could be used by a thread whose primary purpose is to
* serve the queue, but also wants to do other things with its idle time.
*/
bool runPending();
/**
* runOne() runs at most one ready TimedWork item -- zero if none are
* ready. It returns true if the queue remains open, false if the
* queue has been closed.
*/
bool runOne();
/**
* runFor() runs a subset of ready TimedWork items, until the
* timeslice has been exceeded. It returns true if the queue remains
* open, false if the queue has been closed. This could be used by a
* busy main thread to lend a bounded few CPU cycles to this WorkQueue
* without risking the WorkQueue blowing out the length of any one
* frame.
*/
template <typename Rep, typename Period>
bool runFor(const std::chrono::duration<Rep, Period>& timeslice)
{
return runUntil(TimePoint::clock::now() + timeslice);
}
/**
* runUntil() is just like runFor(), only with a specific end time
* instead of a timeslice duration.
*/
bool runUntil(const TimePoint& until);
private:
static void error(const std::string& msg);
static std::string makeName(const std::string& name);
void callWork(const Queue::DataTuple& work);
void callWork(const Work& work);
Queue mQueue;
};
/**
* BackJack is, in effect, a hand-rolled lambda, binding a WorkQueue, a
* CALLABLE that returns bool, a TimePoint and an interval at which to
* relaunch it. As long as the callable continues returning true, BackJack
* keeps resubmitting it to the target WorkQueue.
*/
// Why is BackJack a class and not a lambda? Because, unlike a lambda, a
// class method gets its own 'this' pointer -- which we need to resubmit
// the whole BackJack callable.
template <typename Rep, typename Period, typename CALLABLE>
class WorkQueue::BackJack
{
public:
// bind the desired data
BackJack(WorkQueue::weak_t target,
const WorkQueue::TimePoint& start,
const std::chrono::duration<Rep, Period>& interval,
CALLABLE&& callable):
mTarget(target),
mStart(start),
mInterval(interval),
mCallable(std::move(callable))
{}
// Call by target WorkQueue -- note that although WE require a
// callable returning bool, WorkQueue wants a void callable. We
// consume the bool.
void operator()()
{
// If mCallable() throws an exception, don't catch it here: if it
// throws once, it's likely to throw every time, so it's a waste
// of time to arrange to call it again.
if (mCallable())
{
// Modify mStart to the new start time we desire. If we simply
// added mInterval to now, we'd get actual timings of
// (mInterval + slop), where 'slop' is the latency between the
// previous mStart and the WorkQueue actually calling us.
// Instead, add mInterval to mStart so that at least we
// register our intent to fire at exact mIntervals.
mStart += mInterval;
// We're being called at this moment by the target WorkQueue.
// Assume it still exists, rather than checking the result of
// lock().
// Resubmit the whole *this callable: that's why we're a class
// rather than a lambda. Allow moving *this so we can carry a
// move-only callable; but naturally this statement must be
// the last time we reference this instance, which may become
// moved-from.
try
{
mTarget.lock()->post(mStart, std::move(*this));
}
catch (const Closed&)
{
// Once this queue is closed, oh well, just stop
}
}
}
private:
WorkQueue::weak_t mTarget;
WorkQueue::TimePoint mStart;
std::chrono::duration<Rep, Period> mInterval;
CALLABLE mCallable;
};
template <typename Rep, typename Period, typename CALLABLE>
void WorkQueue::postEvery(const std::chrono::duration<Rep, Period>& interval,
CALLABLE&& callable)
{
if (interval.count() <= 0)
{
// It's essential that postEvery() be called with a positive
// interval, since each call to BackJack posts another instance of
// itself at (start + interval) and we order by target time. A
// zero or negative interval would result in that BackJack
// instance going to the head of the queue every time, immediately
// ready to run. Effectively that would produce an infinite loop,
// a denial of service on this WorkQueue.
error("postEvery(interval) may not be 0");
}
// Instantiate and post a suitable BackJack, binding a weak_ptr to
// self, the current time, the desired interval and the desired
// callable.
post(
BackJack<Rep, Period, CALLABLE>(
getWeak(), TimePoint::clock::now(), interval, std::move(callable)));
}
} // namespace LL
#endif /* ! defined(LL_WORKQUEUE_H) */